Chemical compounds may be identified by spectral techniques, which may be classified broadly by the type of energy that is used. Such techniques may be referred to as “light spectroscopy” when photons are used, “electron” or “Auger” spectroscopy when electrons are used, and “mass spectroscopy” when ions are used. Spectral or spectroscopic techniques in general require an energy source and a device for measuring the change in the energy of the source after the energy has interacted with a sample or chemical compound. Light spectroscopy may be classified by the type of optical light that is produced by the light source, i.e., infrared or “IR” spectroscopy, “visible” spectroscopy, and, ultraviolet or “UV” spectroscopy. For light spectroscopy, lasers and arc lamps are commonly used as energy sources and spectrometers and interferometers are often used as the measuring device.
Depending on the type of energy source used, light spectroscopy may be further classified as being passive or active. Passive light spectroscopy uses the inherent thermal radiance of a material as a radiation source, while active light spectroscopy uses a light source to illuminate a region of interest. Active spectroscopic techniques may detect and measure light that has been reflected or scattered from or transmitted through a target or region that contains a gas, a liquid, or a solid. These techniques include various “absorption” techniques that in general detect the percentage difference between light that is incident on a target or sample and light that is transmitted or reflected from that target or sample.
For absorption spectroscopy, energy in the form of flux photons from a light source is directed to and incident on a sample, region or target. Molecules within the sample, region or target may absorb light at particular frequencies, and different molecules may absorb energy more readily at certain frequencies than at others. Within a given molecule, bonds between atoms or groups of atoms, or “functional groups,” may have characteristic stretching, bending and twisting resonant frequencies. Any given chemical will absorb energy at such resonant frequencies and tend not to absorb energy at other non-resonant frequencies. Light from a source that is incident on and reflected from or transmitted through a chemical will contain an absorption spectrum or “chemical fingerprint” that is characteristic of that particular chemical. Different chemicals have different absorption spectra and characteristic features, e.g., peaks and valleys, may exist across the optical spectrum from the far ultra violet (FUV), through the visible region, to the far or long wave infrared (LW IR). Many chemical absorption spectra are known and such absorption spectra may be found in many chemical literature references.
Absorption spectroscopic techniques can be used to detect chemicals at remote locations because there are certain optical transmission bands or “windows” in the atmosphere. These windows exist because the gas molecules of the atmosphere have their own absorption spectra and absorb light at certain wavelengths and frequencies and not at others. Consequently, certain optical wavelengths are unsuited for absorption spectroscopy while certain others are well suited for such use. For example, infrared (IR) transmission windows exist in wavelength ranges between 3 and 5 microns and between 8 and 12 microns, as well as others.
Active light spectroscopy techniques have been used previously for the detection of chemical agents at a distance. However, these techniques have utilized the tuning of a light source across a wavelength spectrum. Active optical spectroscopy systems that have been used to identify chemicals at a remote location have typically done so by tuning a laser source, e.g., a tunable diode laser (TDL), through a range of wavelengths and by detecting the absorption for each tuned wavelength. The incorporation of such tuning slows the chemical identification process. The necessary tuning across the spectral range of interest takes a period of time and requires additional apparatus of added complexity, size and expense.
Passive light spectroscopy techniques have also been used to detect chemicals at a distance. These are slower than active systems because of the time required to detect thermally emitted radiation or scatted and diffuse light. Additionally, the lack of an illumination source in such systems leads to inconsistent and unreliable light detection and further necessitates detection over long periods of time. The Joint Service Lightweight Standoff Chemical Agent Detector (JSLSCAD) used by the U.S. Armed Forces is an example of such a passive optical spectroscopic system. In addition, passive systems such as the Joint Service Lightweight Standoff Chemical Agent Detector are subject to high false alarms rates due to the low signal-to-noise ratio (S/N) of the detected thermal radiation and dark current noise within the detectors of the passive system.
Nonlinear optics have been used to generate light at certain wavelength ranges in which laser sources are not available, e.g., due to a lack of lasing or active media generating light in those wavelength ranges at sufficient power levels. Nonlinear optics techniques include optical parametric generation, which involves the mixing of three optical waves or fields within a nonlinear crystal. A degeneracy point occurs when two of the three waves have the same or nearly the same energy or frequency (and consequently, wavelength). Information related to attempts to tuning a parametric oscillator through the degeneracy point can be found in U.S. Pat. No. 4,349,907, U.S. Pat. Pub. No. US2002/0176454, and U.S. Pat. Pub. No. US2002/0176472. U.S. Pat. No. 4,349,907 (“'907 patent”) entitled “Broadly Tunable Picosecond IR Source,” published Sep. 14, 1982, discloses a picosecond traveling-wave parametric device capable of controlled spectral bandwidth and wavelength in the infrared region. The output of this system is not broadband in nature but is only tunable over a range of infrared wavelengths. Tuning over such a range is slow and is not suitable for real time or quasi real time spectroscopy.
U.S. Pat. Pub. No. US2002/0176454, entitled “Method for Tuning Nonlinear Frequency Mixing Devices Through Degeneracy,” published Nov. 28, 2002, and related U.S. Pat. Pub. No. US2002/0176472, entitled “Tunable Light Source Employing Optical Parametric Oscillation Near Degeneracy,” published Nov. 28, 2002, disclose the tuning of optical parametric devices through the degeneracy point to achieve an output with a wide tuning range. As with the above-referenced '907 patent, tuning by its very nature takes a certain period of time, and is therefore inherently slower than without tuning. The techniques disclosed are therefore unsuited for real time or quasi real time spectroscopy. Additionally, tuning apparatus are inherently complex and increase the size and cost of the systems they are used with.
What is needed therefore is a real time or quasi real time chemical detection system that is relatively simple and inexpensive. What is further needed is such a system that can qualitatively detect chemical agents including chemical and biological warfare agents at a remote location without the need for tuning across an output range.